Groundwater implications on methane emission from non-sewered sanitation systems in Nepal

Non-sewered sanitation systems (NSSS) are identified as significant contributors of greenhouse gases (GHGs), primarily due to biological processes occurring within the containment systems. In unsealed or unlined containment systems like pit latrines, the emissions are influenced by moisture. This work quantified the GHG emission from unlined or unsealed containments prevalent in Nepal and compared it with sealed containment-like septic tanks, where the chances of groundwater (GW) inundation are low. The modeled GW data extracted from the secondary sources were validated with available national data. The emissions were quantified using the Intergovernmental Panel for Climate Change (IPCC) model for different ecological and provincial divisions of Nepal. Spatial representation for the results was done using the Geographical Information System (GIS) tool. The total methane (CH4) emission occurring from the various NSSS was determined to be 2618 Gg CO2 e per year which is almost twice the emission from the waste sector in 2011, as reported by the recent national communication submitted to the United Nations Framework Convention on Climate Change (UNFCC). Variation of the CH4 emission was found to be prominent in lowlands (Terai region) with total national emissions of 1329.37 Gg CO2e per year. The lowland has a shallow GW table that can easily inundate the unlined containments like pit latrines thus contributing to more anaerobic conditions which may lead to higher CH4 emissions compared to containments in mid and highlands. This study concludes that the GHG emissions occurring from NSSS are substantial and addressing these emissions can help fulfil the Nationally Determined Contributions (NDCs) in the waste sector.